Imagine you are born as a female. There are some complications - you are born without a uterus, but besides that everything seems to be in order. You have a normal childhood. During puberty, your body starts pumping estrogen, causing your breasts to start growing, and you start getting pubic hair around your vagina. But there's something wrong - you never have your first period.
You go to the doctor, and he looks around, does a few tests, and comes back. What he says shocks you - you arn't a girl at all. You are completely, and 100% a man. You have Complete Androgen insensitivity syndrome, a genetic condition that has no effect on women, but on men, it causes their body to develop like a girl. You're mother is shocked, and starts saying some words you have only started learning in science class.
"This is impossible!" she says. "My daughter has a vagina and femine breasts. How could she be a man?".
"Not quite," the doctor says.
He explains that you do in fact have a real vagina. The reason is because, during development, both men and women have a Paramesonephric duct, which later on becomes the vagina. However, most men lose this duct during development. You didn't. Similarily, both men and women have identical breasts. Exposed to enough estrogen, and any man will grow female breasts. So it's suprising that you have feminine breasts.
Your mom still is not satisfied. "If my daughter is a man, then how could she have gone through female puberty?"
The doctor has an answer for this as well. It turns out, he says, that you do not have ovaries - instead, you have two testes inside your body. He explains that testes can actually produce estrogen, likewise, ovaries can produce testoreone. Since your body has a disease which makes it think it is female, it makes your testes think that they are ovaries, and so it produces a lot of estrogen. However, they are still testicles, and you are suseptible to disease a female can never get, such as testicular cancer. It also means you are infertile - your body has no eggs, only little sperm cells which get very confused and die prematurely.
You and your mom are no convinced - there is no doubt that you are male by sex. But could you possibly convince yourself to switch genders? I doubt it. By this point, you have already settled to identifying as a woman. Identifying as a man would feel like you are lying to the world - and based on what we know now about gender, you probably would be. There is nothing wrong with this - your sex simply doesn't match your gender. You're transgender. And you are 100% a woman.
Much more often, people are born with a body that, for the most part, matches their sex. That doesn't mean there is a complete fit - in fact, it is pretty rare for the body to align perfectly with one sex (something like 1 in 10). But for most people, it is pretty easy to guess their sex just by looking at them. However, this doesn't let us know their sex. Sex is how they personally identify. In the above example, you identified as a woman.
Just because gender is how you identify, it doesn't mean you get to choose it. That's actually a pretty ridiculous notion, considering all the pain that transgender people go through, including a suicide (attempt) rate of around 40%. Would you ever say that depression is a choice? Or that autism is a choice? No, of course not. That's ridiculous.
The only choice transgender people make is whether or not they want to be open about their real gender. Caitlyn Jenner never "chose" to be a woman - she has felt like a woman her entire life. What she did choose to do was tell the truth and show us the real her.
Just to recap our ideas that we have discussed up to this point - gender is no more a choice than sex. Therefore, transgender people don't actually change their gender, instead, they were simply born a different gender than their sex. Sometimes, transgender people have a body that naturally aligns with their gender. More often, transgender people have a body that naturally aligns with their sex. Kapich?
Ok, this is where the disagreement comes in.
First, we have to establish what the current consensus is for gender is. There are two genders - male, and female. It is also possible to be neither gender, or agender. Finally, it is possible to be gender fluid - constantly shifting between man and woman.
But according to tumblr, there are a lot more genders, such as:
Aesthetigender - a gender that is derived from an aesthetic
Astralgender - a gender that feels connected to space
Colorgender - a gender associated with one or more colors and the feelings, hues, emotions, and/or objects associated with that color; may be used like pinkgender, bluegender, yellowgender
Espigender - a gender that is related to being a spirit or exists on a higher or extradimensional plane
Now, can we honesly compare these to real genders? Caitlyn Jenner was 66 years old when she came out as a woman. That's 66 years of being called "he" and "him", being called a masculine name, that's seeing male genatalia attached to your body everyday, hell, Caitlyn was at one point paraded around as the symbol of peak masculinity. And yet through all this time Caitlyn never was able to lose her identiy as a female. That's how strong, how unchanging, and uncontrollable gender is.
It's total BS to consider these tumblr genders as remotely comparable to actual genders. I mean there is no doubt that if you intentionally call Caitlyn Jenner "he", you are an asshole. But if you are tired of calling someone by their astralgender pronouns, honestly I can't blame you.